bpf: Add schedule point in htab_init_buckets()

We noticed that with a LOCKDEP enabled kernel,
allocating a hash table with 65536 buckets would
use more than 60ms.

htab_init_buckets() runs from process context,
it is safe to schedule to avoid latency spikes.

Fixes: c50eb518 ("bpf: Use separate lockdep class for each hashtab")
Reported-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201221192506.707584-1-eric.dumazet@gmail.com